An antilocking device for a vehicle brake system is disclosed in Japanese Patent Publication No. 28307/1974, for example, wherein two electromagnetic valves are provided for one wheel so that fluid pressure for a wheel brake is controlled in three modes of pressure application, holding and pressure reduction by controlling the operations of the two electromagnetic valves. Such an antilocking device for a vehicle for a vehicle brake system has recently come into wide use, and it is necessary to reduce the size and cost of such a device. In place of the antilocking device disclosed in the above mentioned Japanese Patent Publication, it is possible to provide a single electromagnetic valve for each wheel and to control the fluid pressure for the wheel brake in two modes of pressure application and pressure reduction by controlling the operation of the electromagnetic valve.
In other words, an antilocking device could be provided with a three-port two-position control valve having three ports which are connected to a pressure source, a wheel brake and a reservoir respectively. Such a antilocking device is so structured that the port which is connected to the pressure source communicates with the port which is connected to the wheel brake when no power is supplied to the three-port two-position control valve while the port which is connected to the wheel brake communicates with the port which is connected to the reservoir when power is supplied to the three-port two-position control valve. When the three-port two-position control valve is not supplied with power, a normal braking operation through a brake pedal and re-pressurization for an antilocking control are implemented. When the three-port two-position control valve is supplied with power, on the other hand, a pressure reduction for antilocking control is implemented.
However, the antilocking device comprising the aforementioned three-port two-position control valve has the following problem: The rate of re-pressurization for the antilocking control must be reduced to assure the necessary controllability. However, when the rate of re-pressurization is reduced by a generally used throttle device, the rate of pressure application in the normal braking operation is also reduced in addition to the rate of re-pressurization for the antilocking control. Thus, it is difficult to achieve an optimal brake application by merely controlling the operation of the three-port two-position control valve.
A control valve employed for an antilocking device must be capable of ensuring a high pressure application rate in a normal braking operation and implementing a small re-pressurization rate in a re-pressurization for the antilocking control. British Patent Publication No. GB8512610 corresponding to U.S. Pat. No. 4,715,666 (Farr) discloses a flow control valve which implements such operations.
FIGS. 5A, 5B and 5C illustrate a flow control valve 3 which is disclosed in U.S. Pat. No. 4,715,666. The flow control valve 3 comprises a frame 31 which has an inlet port 31a connected to a master cylinder 2, an outlet port 31b connected to a wheel brake 4, and an exhaust port 31c connected to an electromagnetic valve 5, which is a two-port two-position control valve. The frame 31 has a cylindrical bore 31g in its interior. This cylindrical bore 31g defines a pressure chamber 35 which is connected to the inlet port 31a on its first end and a pressure reducing chamber 36 which is connected to the outlet port 31b and the exhaust port 31c on its second end. A spool 32 and a spring 34 are contained in the cylindrical bore 31g of the frame 31. The spool 32 has a fluid passage, which extends through the same to connect both ends thereof and defines an orifice 33 in an intermediate position. A first end of the spool 32 is faced with the pressure chamber 35, while its second end is faced with the pressure reducing chamber 36. The spool 32 is so slidable in the cylindrical bore 31g as to switch communications between the ports. The spring 34 urges the spool 32 toward the pressure chamber 35.
When no antilocking control is performed, the flow control valve 3 is in the state shown in FIG. 5A. In this state, a large passage is defined to connect the inlet port 31, an outer peripheral groove portion 32a of the spool 32 and the outlet port 31b.
During pressure reduction for the antilocking control, power is supplied to the electromagnetic valve 5 to open the same. Then, working fluid stored in the pressure reducing chamber 36 is exhausted through the exhaust port 31c to a reservoir 63. Consequently, differential pressure is developed across the spool 32, whereby the spool 32 moves toward the pressure reducing chamber 36 and enters a state shown in FIG. 5B. In the state shown in FIG. 5B, an edge 32b of the spool 32 closes the aforementioned large passage. The spool 32 further moves toward the pressure reducing chamber 36 from the state shown in FIG. 5B, to enter a state shown in FIG. 5C. In the state shown in FIG. 5C, another edge 32c of the spool 32 opens an exhaust passage connecting the outlet port 31b, the outer peripheral groove portion 32a of the spool 32, a passage 31e and the exhaust port 31b. Working fluid applying pressure to the wheel brake 4 is exhausted to the reservoir 63 through the aforementioned exhaust passage and the electromagnetic valve 5. Thus, fluid pressure for the wheel brake 4 is reduced. The working fluid stored in the reservoir 63 is absorbed and pressurized by a pump 61, which is driven by a motor 62, to be fed back between the master cylinder 2 and the inlet port 31a.
When no power is supplied to the electromagnetic valve 5 in re-pressurization for antilocking control, a variable orifice is defined by an edge portion 32d of the spool 32 and the inner peripheral end of a passage 31f in the state shown in FIG. 5C to define a small passage connecting the inlet port 31a, a passage 31d, the pressure chamber 35, the orifice 33, the pressure reducing chamber 36, the passages 31f and 31e, the outer peripheral groove portion 32a of the spool 32, and the outlet port 31b, thereby loosely increasing the fluid pressure for the wheel brake 4. When differential pressure across the inlet and outlet ports 31a and 31b is reduced, the spool 32 returns to its original position, to attain the state shown in FIG. 5A.
The antilocking device shown in FIG. 5A has a cast advantage since only one electromagnetic valve is required for each wheel. During the re-pressurization for an antilocking control in the state shown in FIG. 5C, the degree of opening of the variable orifice defined by the edge portion 32d of the spool 32 and the inner peripheral end of the passage 31f, is automatically adjusted so that a differential pressure developed across the fixed orifice 33 by a flow of the fluid passing through the fixed orifice 33, is balanced by pressure which is determined by the effective sectional area of the spool 32 and by the urging force of the spring 34. Therefore, the flow rate of the working fluid during re-pressurization is constant regardless of the value of the differential pressure across the inlet port 31a and the outlet port 31b. Further, since the differential pressure across the fixed orifice 33 can be reduced, it is possible to ensure a small flow rate even if the fixed orifice 33 has a relatively large diameter, whereby the antilocking device can be easily applied to a small vehicle having a small brake with a small fluid consumption.
However, the flow control valve disclosed in U.S. Pat. No. 4,715,666 has the following problems:
In the flow control valve shown in FIG. 5A, a small passage closing member is provided in the passage connecting the pressure reducing chamber 36 with the outlet port 31b. This small passage closing member is defined by the edge 32c of the spool 32 and the wall surface of the cylindrical bore 31g. In the flow control valve having such a structure, the small passage connecting the pressure reducing chamber 36 with the outlet port 31b must be opened after the large passage connecting the inlet port 31a with the outlet port 31b is closed for a pressure reduction for an antilocking control. If the small passage connecting the pressure reducing chamber 36 with the outlet port 31b is opened before the large passage connecting the inlet port 31a with the outlet port 31b is closed, the working fluid introduced into the inlet port 31a is guided to the pressure reducing chamber 36 through the large passage by way of the small passage closing member. Thus, no differential pressure is developed across the spool 32, which enters a stationary state to achieve no pressure reduction. In the flow control valve 3 shown in FIG. 5A, therefore, it is necessary to open the small passage connecting the pressure reducing chamber 36 with the outlet port 31b after the large passage connecting the inlet port 31a with the outlet port 31b is closed.
The frame 31 and the spool 32 must be operated for implementing the aforementioned desired operation. In this case, the frame 31 and the spool 32 are to be operated in consideration of dimensional manufacturing errors. Such an operation causes the small passage connecting the pressure reducing chamber 36 with the outlet port 31b to be closed at the same time when the large passage connecting the inlet port 31a with the outlet port 31b is closed, as shown in FIG. 5B. If the spool 32 is fixed in a stationary state by contamination of foreign matters, for example, all passages connecting the inlet port 31a with the outlet port 31b are closed. Such a state is not desired in view of safety considerations since no pressure can be applied to the wheel brake 4.
Further, the flow control valve shown in FIG. 5A requires the small passage closing member defined by the edge 32c of the spool 32 and the wall surface of the cylindrical bore 31g, for closing the small passage connecting the pressure reducing chamber 36 with the outlet port 31b. Thus, the passages defined in the frame 31 are complicated in structure, whereby costs are increased. For example, the flow control valve 3 shown in FIG. 5A requires the passages 31e and 31f.